Test results!
 

I had to jump start the car this afternoon. I apparently didn't get the battery sufficiently charged yesterday after jump starting it. Once the car was jump started today, I took these readings from the battery (while the engine was running at idle):
12.94 volts
5.88 amps

Next, I drove the car at about 65 MPH for about 12 miles.

When I arrived home, I took these readings from the battery (while the engine was running at idle):
12.58 volts
5.67 amps

I shut the engine off and waited about 30 seconds.

I started the engine AND AT IDLE, THE CHARGING LAMP WAS OFF. This has *not* been the case for many months, if not years.

Once again, I shut the engine off and waited about 30 seconds.
Once again, I started the engine AND AT IDLE, THE CHARGING LAMP WAS OFF.

Weird.

I shut the engine off and waited about 20 minutes.
I started the engine AND AT IDLE, THE CHARGING LAMP WAS ON!
This is how this car has worked for years.

-tombance-
Battery readings as you requested:
Car switched off: 12.19 volts, 5.44 amps
Car started, charging lamp on at first idle: 11.87 volts, 5.27 amps
Car started, just after I rev the engine so that the charging lamp goes out: 12.26 volts, 5.57 amps
Car started, transmission in neutral and engine at 2,500 RPMs: 13.23 volts, 6.1 amps

Any thoughts about these results?

Thanks-

your alternator is weak, it should be able to put out about 13V at idle with everything off.

Remove the voltage regulator on the alternator and inspect the brushes.

That's what I was thinking, but ONLY because it is very easy to do, and the alternator is 8 years old I believe the OP stated.

My bad on the location of the external fuse - it's on top of the LEFT front fender, a short cable run from the fuse box. Small black plastic holder maybe two inches long by less than an inch wide.

Also see the following thread for more info.
http://www.peachparts.com/shopforum/showthread.php?t=255363&highlight=blower+motor+external+fuse

It's probably just gee-whiz info at this point, though, since it's probably NOT causing your problem.

But the measurements for current on your car sound off. If your car was idling and the battery was charging there had to be more than 5.5 amps or so going to the battery.

Also, the same 5 amp draw with everything off is WAY high. It should be low milliamps. There's a more professional write-up on battery draw down testing here: http://www.aa1car.com/library/battery_runs_down.htm

It also describes a specific fault with a diode in the alternator. Your Fluke DMM should have an AC setting, so you may be able to look at that.

Checking potential alternator issues
 

Thanks for the follow-up analysis of the results.
I also suspected a charging issue given the readings I collected.

79Mercy
"Remove the voltage regulator on the alternator and inspect the brushes."
***It has been years since I looked at the back of the alternator...Is the voltage regulator right at where the connector fits into the alternator?

---

Yak
"Also, the same 5 amp draw with everything off is WAY high. It should be low milliamps. There's a more professional write-up on battery draw down testing here: http://www.aa1car.com/library/battery_runs_down.htm"
***It certainly seems like something is pulling a significant amount of energy out of the battery.

That's a very useful battery draw down testing write-up.

Regarding the bad alternator diode description in the battery draw down testing write-up...
Q: On the last page where they refer to using the AC scale (which my Fluke DMM definitely has)...
Q1: Are my two DMM leads set between one of the disconnected battery cables and that cable's post (from step 3. under To measure current drain without an inductive amp probe:)?
Q2: This is to be done with the engine running, correct?
Q3: Is it ok to start the engine, disconnect one of the cables (doesn't matter which one?) and then do Q1 above?

Thanks again for all of the help with tracking down the problem(s), fellas-

The voltage regulator is attached to the alternator with two screws. I used a stubby phillips to remove them. Here is diesel giant's nice write-up with pics.

http://www.dieselgiant.com/mercedesvoltageregulatorinstall.htm

As 79Mercy said, there is something very wrong with your alternator, replacing the voltage regulator on the back will probably solve the issue.

However, I dont think you should ever disconnect the battery cable with the engine running, as it can cause high voltage spikes that can damage things. Thankfully, these cars dont have computers to be damaged (and have an OVP?) and it doesnt sound like your alternator is working much anyway. You really want to do the same AMP readiong with the engine switched off, all accessories turned off (just as you would leave the car parked overnight). The current draw in this situation should be very low (like 0.01A). if it is not you have something draining your battery as well.

Alternator voltage regulator
 

1stimer
Thanks for the link. A perfectly clear write-up and seemingly a very easy job.

I am still suspicious that a bad alternator diode may be causing substantial draining of the battery. Even if the two carbon stacks on the voltage regulator are substantially worn (which I doubt given the mileage on the alternator), I do not think that that would account for the 5 amp draw with everything off and the engine running. As Yak said, the 5 amp draw with everything off is WAY high. If that's the case, the alternator apparently gets replaced.

Checking amps with engine and all accessories turned off
 

tombance
"You really want to do the same AMP readiong with the engine switched off, all accessories turned off (just as you would leave the car parked overnight). The current draw in this situation should be very low (like 0.01A). if it is not you have something draining your battery as well."
I'll do that later today when I'm back at home with access to the vehicle. I'll post the results.

Thanks-

Danger! Don't disconnect cables with the car running!
 

Don't try connecting or disconnecting battery cables while the engine is running. Bad, dangerous, etc.

For checking to see if there's an alternating current component caused by a bad diode, you should just be able to leave your car set up for normal operation with the engine running, change the scale on the DMM to AC volts (maybe a picture of a sine wave) and see if it shows any AC voltage at your battery. If so, then at least according to that web page, then you've got a bad diode that allows electricity to "leak" back out of the battery.

Think of voltage as the strength of the electricity and current as the flow. You might get a high strength, and with DC it's going into your battery from the alternator; if there's an AC component it might be just as strong (same voltage) but sometimes it goes into the battery and sometimes it goes out. The diode is supposed to stop that outflow.

Since I'm not really doing anything else, and I'm waiting to register my car tomorrow, and I've basically got the same car; I'll take some measurements and some photos and tell you what I get for "parasitic ignition off current drain" and charging voltages.

Disclaimer: I have no historical data on my car since I've only owned it for 2 months and haven't driven it except into my driveway. But, the battery is new and it has started every time I've rolled it into or out of the garage, which is maybe a dozen times. I'll assume it has normal charging capacity and normal parasitic drain.

Gimme a couple hours and I'll post with pics and data.

Meanwhile - the same page has an equally good write-up on how to test an alternator. http://www.aa1car.com/library/charging_checks.htm

The link about rebuilt parts is good, too. I think it recommends having a shop bench test your "new" alternator before you install it. Not because they're frequently bad, but because they have the highest return rate:

"When rebuilding an alternator, for example, one rebuilder may replace diodes only if a failure has occurred. But another may automatically replace all the diodes with ones that are more durable to improve reliability. One rebuilder may do nothing to the armature commutator other than to clean it up while another always recuts the commutator on a lathe to minimize runout for smooth high rpm operation. One rebuilder may reuse slip rings while another replaces them or resurfaces them on a lathe to minimize runout. Some rebuilders do not full-load test their alternators for fear of damaging their units while others will always full-load test each and every unit to make sure they meet specs and won't fail."

Got it: Keep cables connected when engine running!
 

Thanks for the clarification and explanations, Yak. It helps a lot.

I'll just run the engine as normal, set the DMM to AC, connect one DMM lead to each battery post, and check for an AC reading.

As I previously wrote, the alternator is 8 years old. When installed, it was a Bosch rebuild acquired from Phil here on Fastlane. As your post noted, there is variation in what is done when rebuilding just as there is variation in testing. An area of uncertainty.

Also, as I wrote previously, I had the alternator checked at a local shop a while back (not recently). They said it was ok. However, perhaps the problem has been intermittent. Perhaps their testing didn't check for a faulty diode. Hard to say.

Does anybody know...
Is the diode located inside the voltage regulator (and thus can be eliminated by replacing the voltage regulator) or is it farther into the interior of the alternator and thus, if found faulty, necessitates replacement of the alternator as a unit?

Later today, I'll check for the "parasitic ignition off current drain" and an AC reading as stated above and post here.

Thank you-

Car: 1983 300CD
Battery: new (<3 mos old) Interstate MTP-93
Alternator: unknown
Measurement tool: Greenlee DM-200 digital multimeter, standard pointy probes
Conditions: 90 degrees, car has not run or been started since yesterday, nor has it been allowed to run more than 20 minutes in the past two months, some dielectric paste on the terminals. I suspect the battery is a tad bit lower than normal.

Car off, nothing turned on, both terminals connected to car, voltage measured from post to post: 12.05 V
Car off, nothing turned on, negative terminal disconnected, voltage measured post to post: 12.05 V
Car off, nothing turned on, negative terminal disconnected, DMM set to A, current measured from negative terminal to negative post: 20 milliamps (This is the "parasitic draw" that occurs overnight).
Car off, trunk open (for reference current draw for a bulb) negative terminal disconnected, DMM set to A, current measured with probes from negative terminal to negative post: 700 milliamps (or 0.7 Amps displayed)
Car off, trunk closed but right "standlight" turned on, same conditions above: 1.2 A.

Car on, both terminals connected, alternator working okay, engine at idle, voltage measured post to post: 13.45 v DC and for grins I went to AC and measured 0.05 v AC, too. I suppose I should have checked that without the alternator running or connected just to see if that's some anomalous input into the DMM or if that really came from the alternator.

So, comparing the measurements to dogguy's; if he does indeed have a 5 Amp draw when everything is turned off, that's huge! Like 250 times what my engine off current drain is. Voltages are similar, I can't say whether I've really got a bad diode with a minor AC component.

My recommendation is to double check your parasitic drain to ensure it's not 5 Amps, since that number is really too consistent across your measurements to make much sense. But if it really is, then it's time to start removing fuses one at a time to see if you can find the specific source.

Also, if you really show an AC component from the alternator, then I'd recommend getting it tested at a shop and specifically ask if they can test for a bad diode.

I didn't do the voltage drop from alternator to battery as recommended on the "test an alternator" site since I'm not familiar enough with the alternator, but that would provide an indication of the condition of the wires going to the battery. The alternator "might" be okay, it may just have bad connections.

Note: My DMM was acting a bit funky in current mode. I couldn't get any measurement in the milli-amp setting, so I may have a blown fuse there (0.25 A max). I was also initially showing zero Amps for parasitic draw, so I opened the trunk to kick on the light bulb and force a current draw. It went to 700 mA, then when I shut the trunk the DMM read a steady 20 mA. I can't explain why it didn't measure correctly to begin with. Maybe I didn't have a good connection with the probe, or maybe the meter needed a larger to current to "wake up" since I haven't used the ammeter mode or ports in years.

More test results
 

I had to jump start the battery again to get the engine running.

Once I had the engine running, the charging lamp stayed on as has been the pattern for years. With the lamp on, I measured as follows:
0.000 volts AC, 13.09 volts DC

I reved the engine once and the charging lamp went out as has been the pattern for years. With the lamp off, I measured as follows:
0.053 volts AC (didn't get the volts DC, unfortunately)

Is this then an indicator of a diode fault?

---

***Car off, nothing turned on, both terminals connected to car, voltage measured post to post: 12.02 volts
***Car off, nothing turned on,negative terminal disconnected, voltage measured post to post: 12.04 volts

Yak
"Car off, nothing turned on, negative terminal disconnected, DMM set to A, current measured from negative terminal to negative post: 20 milliamps (This is the "parasitic draw" that occurs overnight)."
My ignorance: I simply do not understand what this means. Is this to mean that the negative battery cable is removed from the negative battery post and that you have one DMM lead on the negative battery cable and the other on the negative battery post? If so, I measure 0.00 with this arrangement. Then again, my DMM only has a 10 amp setting so perhaps this is too small of a quantity to measure.

I think I do not understand how you are measuring milliamps. My ignorance.

Thanks-

You answered your problem. The charging volts should between 13-14 volts especially when revving the engine. You wont get DC since the altenator takes DC current changes it to AC through the rectifier bridge.

That's exactly what I had when I first measured it - zero current, but I knew it couldn't be correct since the radio draws some tiny fraction of power to to run its internal clock, as does the clock in the dash - see the note at the bottom of my post.

You could try again and do something to trigger a slightly larger current draw, but not too much, that's why I opened my trunk. I wanted to make sure that the meter was actually reading something. With the trunk open and the light inside it on, the DMM displayed 0.700, so I closed my trunk and it went down to 0.02. Then I turned on the one side parking lights and it went to 1.2, turned them off and it went back to 0.02. These seemed to be normal predictable current readings, so I assumed good values from here out.

My conclusion was that my "off" condition for my car would draw 0.02 amps, or 20 milliamps; which is about predictable for a car with nothing turned on. I don't know why my DMM wouldn't measure that from the start, though.

You've probably over-confirmed that you've got a bad alternator and maybe confirmed that you probably DON'T have any excessive draw down.

I'd recommend getting it tested by a shop that can describe what they're testing and what the results mean.